Process for making fluorescent material



Patented Feb. 23, 1943 UNITED STATES PATENT omca PROCESS FOR MAKINGFLUORESCENT MATERIAL Willard A. Roberts, Euclid, Ohio, assignor toGeneral Electric Company, a corporation of ..New

York

No Drawing. Application January 16, 1941, Serial No. 314,731

11 Claims. (CL 250-81) pound of the base metal involved, considerablevariation in brightness and uniformity of the resultant fluorescentmaterial is experienced with different batches of the oxide. 4 Thepurification of the tungstic oxide has not greatly improved the results.The cause of the phenomenoncan probably be traced to residues of theoriginal ingredients remaining in the end product of the reaction. It isknown that excesses of certainof these materials sometimes havedeleterious effects upon the brightness of .the fluorescent product. Ihave found that the brightness can reaction in excess, it is insuredthat substantially be greatly improved and uniform results obtained withalmost all batches by using an excess of some of the originalingredients and by adding materials which will remove the harmfulconstituents by driving the reaction to completion.

It is accordingly an object of the invention to provide a process forimproving the brightness and uniformity of the fluorescent tungstates.It

is a further object of the invention to improve the brightness anduniformity of such tungstates by insuring the completion of the reactionbetween the initial ingredients of the process. Further objects andadvantages of the invention will appear from the following detaileddescription thereof.

In one method of preparing lead-activated calcium tungstate, an intimatemixture of calcium oxide, tungstic oxide and lead acetate is fired forfour hours, more or less, at temperatures of the order of 1000 C. Sincethe reaction is to some extent reversible, certain, amounts of calciumoxide and tungstlc oxide tend to remain in the end product withattendant deleterious effect on all the tungstic oxide combines withsome of the calcium oxide to form the desired tungstate. The freetungstic oxide together with its harmful effacts are thus removed. Ihave also found that the excess calcium oxide may be removed by theaddition of a certain amount of sulphuric acid which tends to convertthe excess oxide into the sulphate. The essential requirement is thatthere exist an excess of calcium in the form of the sulphate. Tests haveshown that the addition of from 5 to 10 per cent of the sulphuric acidto the mixture before firing will produce a phosphor of good brightnessfrom all lots of pure tungstic oxide. The amount of the sulphuric acidused is about 25 per cent of that required to change the calcium oxldetothe calcium sulphate. The net result of both the foregoing steps is thatsubstantially all free tungstic oxide and calcium oxide is removed fromthe final product.

While I do not wish to state it as a positive fact, there is someindication that the calcium hate which remains in excess will combinewith any excess lead acetate to form a lead activated calcium sulphatehaving a slight fluorescent output in the blue region. This, of course,enhances the blue output of the calcium tungstate and may be mentionedas a further the brightness of the fluorescent material. It is probablethat the varying amounts of these remaining oxides cause the differencein brightness which is experienced in different batches as alreadymentioned, Excess tungstic oxide is known to produce a poor anddiscolored phosphor. On

the other hand, any calcium oxide left over in I advantage of theinvention. I have also found some evidence to indicate that some of thecalcium sulphate is activated by the tungstic oxide itself to giveadditional fluorescence and to further add to the cleanup of thetungstic oxide. It is known that, by itself, calcium sulphate activatedby tungstic oxide is phosphorescent. For example, 1 per cent of tungsticoxide in calcium sulphate gives a fairly bright phosphor ofapproximately same color as that of calcium tungstate.

Somewhat the same result may be obtained if instead of adding sulphuricacid, calcium sulphate in excess is used as the original ingredient inplace of the calcium oxide. However, in this case, the heating then maybe longer and the powder not so bright. The following table will serveto illustrate the improvement in the relative brightness obtained byusing the process of the invention with different lots A, B, C, D and Eof tungstic oxide for mixtures of various compositions by relativ numberof parts. including the case where the sulphates alone are used and thefirst described case where sulphuric acid is added to the oxide mixture.

Relative fluorescent brightness Lot W;

A A B C D E W0; unfired before mixing calcium ox- .L.. V (7% tungsticox-' (1.8) lead aoetate) 100 76 95 105 107 +8sulphurieacid. 115 113 112113 116 115 (61) calcium sul- I H phate) (75) tungstic ox ide) 110 110(1.8) lead acetate)- +8 sulphuric acid. no 111 (35) calcium car- Ibonate) (75) tungstic ox id 88 92 (1.8 lead acetate) +8 sulphuric acid-112 113 as calcium oxide) (75) tungstic ox (1 8 lead-i 9 lead su lide)+8 sulphuric acid 114 113 "The same improved results may be obtained inthe case of other phosphors by applying the principles oi the invention.For example, in the manufacture of strontium tungstate, as disclosed inmy co-pending application No. 874,739 filed January 16,1941 thebrightness anduniformity of the product. may be improved by the additionof sulphuric acid or other sulphates 'as outlined above. Here, as above,the excess strontium 0xlde drives the reaction with the tungstic oxideto tion herein oi the theory of the process but only by the manipulationthereof as described. Further. it will be understood'that manyvariations in the relative proportionsand perhaps in the materialsthemselves'may occur 'to those skilled in the art to which the inventionappertains. All

such I aim to include within the cope of the appended claims. j

What I claim as new and'desir'e to secure by Letters Patent of thewaited States is:

iu'rne method of producing-a tungstate phosphor which comprises firing amixture of ma- ;prises the step of adding a quantity of from 5 to 10 percent of sulphuric acid to a mixture of tungstlc oxide, an excess of ametallic "oxide and an activator and then firing the mixture.

5. The method of improving the brightness and uniformity of a tungstatephosphor which comprises the step of adding to a mixture of tungsticoxide, an excess of a metallic oxide and an activator before firing, aquantity of sulphuric acid equal to about 25 per cent to that necessaryto convert the calcium oxide to calcium sulphate, and then firing themixture.

6. The method of producing a tungstate phosi phor which comprises thesteps of intimately ad.-

mixing a quantity ,of tungstic oxide, a metallic sulphate in excess ofthe amount required to form the tungstate and an activator and firingsaid admixture.

, '7. The method of producing a tungstate phosphor which comprises thesteps of intimately admixing a quantity of tungstic oxide, a metallicoxide in excess or the amount required to form the tungstate and anactivator and adding to said admixture a quantity of sulphuric acid, andthen firing the mixture.

8. Themethod of producing a tungstate phosphor which comprises the stepsof intimately admixing a quantity of 'tungstic oxide, a metallic oxidein excess of the amount required to form a the, tungstate and anactivator and adding to said admixture a quantity of about 5 to 10 percent ofsulphuric acid, and then firing the mixture.

9. The method of producing a tungstat phosphor which comprises the stepsof intimately admixing. a quantity or tungstic oxide; a metallic oxidein excess of the amount required to form the tungstate and an activatorand adding to said admixture a quantity of sulphuric acid equal to about25 per cent of that necessary to convert the metallic oxide to thesulphate, and then firing the mixture.

ter ials, including tungstic oxide and an excess of a compound oi a basenietalrwhich will react to leave'a fluorescent tungstate of the basemetal and a sulphate of the base metal. 2. 1"he method of improthejbrightness and u i t r a m e h s h in which tungstic oxide and anexcess-'qf ametailic oxide 10. A fluorescent material comprising atungstate of a base metal containing an excess of the tungstatecontaining an excess of calcium in the iorm of calcium sulphate.

' wm 1' A. seem-is.

